In recent years, AC3 (Audio Code number 3), ATRAC (Adaptive TRansform Acoustic Coding), AAC (Advanced Audio Coding), and so forth, which realize high sound quality, have been used as schemes for encoding audio signals. Moreover, audio signals of multiple channels such as 7.1 channels or 5.1 channels have been used to reconstruct a real acoustic effect.
When the audio signals of the multiple channels such as 7.1 channels or 5.1 channels are reproduced with a stereo audio apparatus, the process for downmixing the multi-channel audio signals to stereo audio signals is performed.
For example, when encoded 5.1-channel audio signals are downmixed to reproduce the downmixed audio signals with the stereo audio apparatus, first, a decoding process is performed to generate decoded 5-channel audio signals of a left channel, a right channel, a center channel, a left surround channel, and a right surround channel. Subsequently, in order to generate a stereo left-channel audio signal, respective audio signals of the left channel, the center channel, and the left surround channel are multiplied by mixture ratio coefficients and a summation of the multiplication results is performed. In order to generate a stereo right-channel audio signal, respective audio signals of the right channel, the center channel, and the right surround channel are subjected to the multiplication and the summation, similarly.    Patent Citation 1:
Japanese Unexamined Patent Application, First Publication No. 2000-276196